Recently, with development of information-oriented society, a higher energy density and a higher output density have been demanded for power sources of small-sized portable electronic equipment, such as cellular phones and notebook type personal computers, and electric cars which will take an important role in transportation means in future. While needs for energy storage devices will continue to increase in future, the characteristics much superior to existing ones will be demanded for the energy storage devices at the same time. Main examples of currently available energy storage devices are an electrolytic capacitor, an electric double layer capacitor, and a battery. On the other hand, as novel devices capable of covering the range of power energy density which cannot be achieved with those devices, attention is focused on a new type energy device, called an electrochemical capacitor, in which conducting polymers and metal oxides are used as electrode materials.
An electrochemical capacitor stores and releases energy through a peculiar mechanism different from those of conventional capacitors and electric cells. Therefore, the electrochemical capacitor is expected to have such primary advantageous characteristics that a higher power density is achieved and a larger energy density as compared with the electrolytic capacitor, etc. is realized while ensuring superior cycle characteristics.
There are three kinds of materials for use in the electrochemical capacitor, i.e., carbon-base materials, conductive high-molecular (polymer) materials, and metal oxide-base materials.
Among the carbon-base materials, activated charcoal is used as an electrode material of an electric double layer capacitor. The reason is that the activated charcoal has a high specific surface area and, therefore, a very large electric double layer capacity is developed at the hetero interface between the surface of the activated charcoal and an electrolytic solution. Other examples of carbon materials include carbon black, graphite, carbon nano-tubes, and carbon coils. Of these materials, the carbon black is used as conductivity aids for various conductive materials because of having a high electronic conductivity.
As examples of the metal oxide-base materials, oxides of rare metals, valve metals, transition metals, and lithium transition metals are used as energy storage materials. Of those examples, the valve metals are used as materials of electrolytic capacitors, and the other metals are used as electrode materials of electrochemical capacitors and various electric cells.
Electric double layer capacitors have been employed as small-sized and highly-reliable memory backup power sources from the second-half of 1970s with miniaturization of electronic equipment and introduction of microcomputers and semiconductor memories. At present, electric double layer capacitors are widely used in the energy backup field as aids for secondary cells or new energy supply sources which are expected to be substituted for cells.
An electric double layer is a thin film which is naturally produced at the interface between an electrode and an electrolytic solution, and has a thickness of about several angstroms. When an electrode is immersed in an electrolytic solution and a voltage is applied, no current flows in an initial stage before the start of electrolysis because an electric double layer produced between the electrode and the electrolytic solution functions to maintain insulation. In such a condition, electric charges or holes present on the conductor side with respect to the electric double layer cause adsorption with and desorption from ions having opposite polarities (i.e., non-Faraday reaction between the electrode and the ions without transfer of electrons) so that electric charges are stored and released. The electric double layer capacitor utilizes such a phenomenon. Features of the electric double layer capacitor are as follows: i) quick charging/discharging of a large current is enabled because the charging/discharging accompanies with no chemical reactions, ii) stable charging/discharging behaviors are ensured over a wide temperature range, iii) the capacitor is not failed upon short-circuiting and, therefore, no restrictions are imposed on the charging/discharging, iv) the charging/discharging can be continued semi-permanently and replacement is virtually not required, and v) the capacitor is friendly to environments because of no use of heavy metals, such as lead and cadmium.
On the other hand, when an aqueous-base electrolytic solution, e.g., sulfuric acid, is used, the withstand voltage is as low as 1 V. The withstand voltage can be increased to a level of 3 V by employing an organic electrolytic solution. However, such a capacitor has a problem such as a drop of electronic conductivity and cannot be said as being satisfactory from the viewpoint of energy density. Also, while the electric double layer has a very small capacity of about 20 to 40 μFcm−2 with the use of a mercury electrode, a large capacity of farad order can be obtained by employing, as an electrode, phenol-base activated charcoal fibers or pitch-base activated charcoal powders, which have a large specific surface area with a pore size of a not-too-small level (about 1,500 to 3,000 m2/g), and which are highly conductive and are electrochemically stable.
Electrochemical capacitors using metal oxides are advantageous in that they are more compact, have smaller internal resistance, and have less risk of ignition than the case of using carbon-base materials. Further, an electrochemical capacitor using RuO2 shows relatively high characteristic values, i.e., operating voltage of 1.4 V, energy density of about 101 to 102 Wh/kg, and power density of about 500 W/kg, when immersed in an aqueous solution of sulfuric acid. Thus, a higher energy density than the case of using carbon-base materials is obtained. In consideration of that Ru and Ir are produced in very small amount and are very expensive materials, however, it is studied to use, instead of Ru and Ir, transition metals such as Co and Ni, valve metal oxides such as TiO2, SnO2 and ZrO2, or mixed materials such as V2O5 and RuO2.
The charge storage mechanism of an electrochemical capacitor resides in absorption and desorption of hydrogen ions in the electric double layer and at the electrode surface, as well as incidental reduction and oxidation of metals. Charge accumulation occurs primarily based on the Faraday reaction in which electrons drift at the electrode interface. The valence of a metal in the oxide is changed with reduction and oxidation. In RuO2, for example, the valence of Ru as a center metal continuously changes from 2 to 4, and the reaction expressed by the following formula (1) occurs with motion of protons for compensating for the charge change: